This invention relates to redundancy in voice and data communications systems.
An element of a communications system such as a wireless communication system can fail and can require replacement. The failure of the element and the replacement of the element typically require shutdown of the system, which can be highly expensive and time-consuming due to the long time intervals necessary to bring the system to a stable condition following the shutdown.
If there is no redundancy in the event that the element becomes disabled, or otherwise fails to perform its tasks, the performance and reliability of communications can be compromised. One approach to redundancy is to designate a master element and one or more candidates that would take over as master in a sequential fashion when the existing master fails. That method, although useful for its intended purposes in providing some degree of redundancy, can be cumbersome to configure. The master and the candidate elements may have to be manually selected when the communications system is set up. Another disadvantage is that, once all the candidate elements are exhausted due to multiple failures, the network would again be without a master, causing deterioration or disruption in communications such as wireless communication.
Although the particular application may vary, the components of a wireless communication system are generally similar, as described in more detail below. For example, a wireless communication system usually includes a radio terminal or mobile station, a radio base station, a switch or network control device, often referred to as a mobile telephone switching office (MTSO), and a network to which the wireless communications system provides access, such as the Public Switched Telephone Network (PSTN).
The various wireless communication applications use any of multiple modulation techniques for transmitting information to efficiently utilize the available frequency spectrum. For example, frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access modulation techniques are used to build high-capacity multiple access systems. Telecommunication systems designed to communicate with many mobile stations occupying a common radio spectrum are referred to as multiple access systems.
For example, in an FDMA analog cellular system, such as an AMPS analog cellular radio system, the available frequency spectrum is divided into a large number of radio channels, e.g., pairs of transmit and receive carrier frequencies, each of which corresponds to a message transmission channel. The bandwidth of each transmit and receive frequency channel is narrowband, generally 25-30 kHz. Thus, the FDMA system permits information to be transmitted in a bandwidth comparable to the bandwidth of the transmitted information, such as a voice signal. The cellular service area in the FDMA system is generally divided into multiple cells, each cell having a set of frequency channels selected so as to help reduce co-channel interference between cells.
Frequency division is often combined with time division so that transmission circuits are distinguished in both the frequency and time domain, e.g., in a FD/TDMA system. In a digital FD/TDMA (commonly referred to as TDMA) cellular system, a narrowband frequency channel is reformatted as a digital transmission path which is divided into a number of time slots. The data signals from different calls are interleaved into assigned time slots and sent out with a correspondingly higher bit rate, the time slot assigned to each mobile station being periodically repeated. Although the TDMA bandwidth may be somewhat larger than the FDMA bandwidth, a bandwidth of approximately 30 kHz is generally used for AMPS-D digital TDMA cellular systems.
Another approach to cellular multiple access modulation is CDMA. CDMA is a spread spectrum technique for transmitting information over a wireless communication system in which the bandwidth occupied by the transmitted signal is significantly greater than the bandwidth required by the baseband information signal (e.g., the voice signal). Thus, CDMA modulation spectrally spreads a narrowband information signal over a broad bandwidth by multiplex modulation, using a codeword to identify various signals sharing the same frequency channel. Recognition of the transmitted signal takes place by selecting the spectrally-coded signals using the appropriate codeword. In contrast to the narrowband channels of approximately 30 kHz used in FDMA and TDMA modulation techniques, a CDMA system generally employs a bandwidth of approximately 1.25 MHz or greater.
Typically, the mobile communication systems described above are arranged hierarchically such that a geographical “coverage area” is partitioned into a number of smaller geographical areas called “cells.” Referring to FIG. 1, each cell is preferably served by a Base Transceiver Station (“BTS”) 102a. Several BTS 102a-n are centrally administered via fixed links 104a-n by a Base Station Controller (“BSC”) 106a. The BTSs and BSC are sometimes collectively referred to as the Base Station Subsystem (“BS”) 107. Several BSCs 106b-n may be centrally administered by a Mobile Switching Center (“MSC”) 110 via fixed links 108a-n. 
MSC 110 acts as a local switching exchange (with additional features to handle mobility management requirements, discussed below) and communicates with the phone network (“PSTN”) 120 through trunk groups. U.S. mobile networks include a home MSC and a serving MSC. The home MSC is the MSC corresponding to the exchange associated with a Mobile Subscriber (also referred to above as a mobile station or “MS”) 114; this association is based on the phone number, such as the area code, of the MS. Examples of an MS include a hand-held device such as a mobile phone, a PDA, a 2-way pager, or a laptop computer, or Mobile Unit Equipment, such as a mobile unit attached to a refrigerator van or a rail car, a container, or a trailer.
The home MSC is responsible for a Home Location Register (“HLR”) 118 discussed below. The serving MSC, on the other hand, is the exchange used to connect the MS call to the PSTN. Consequently, sometimes the home MSC and serving MSC functions are served by the same entity, but other times they are not (such as when the MS is roaming). Typically, a Visiting Location Register (“VLR”) 116 is co-located with the MSC 110 and a logically singular HLR is used in the mobile network (a logically singular HLR may be physically distributed but is treated as a single entity). As will be explained below, the HLR and VLR are used for storing subscriber information and profiles.
Radio channels 112 are associated with the entire coverage area. As described above, the radio channels are partitioned into groups of channels allocated to individual cells. The channels are used to carry signaling information to establish call connections and related arrangements, and to carry voice or data information once a call connection is established.
Mobile network signaling has at least two significant aspects. One aspect involves the signaling between an MS and the rest of the network. In the case of 2G (“2G” is the industry term used for “second generation”) and later technology, this signaling concerns access methods used by the MS (such as TDMA or CDMA), pertaining to, for example, assignment of radio channels and authentication. A second aspect involves the signaling among the various entities in the mobile network, such as the signaling among the MSCs, BSCs, VLRs, and HLRs. This second part is sometimes referred to as the Mobile Application Part (“MAP”) especially when used in the context of Signaling System No. 7 (“SS7”). SS7 is a common channel signaling system by which elements of the telephone network exchange information, in the form of messages.
The various forms of signaling (as well as the data and voice communication) are transmitted and received in accordance with various standards. For example, the
Electronics Industries Association (“EIA”) and Telecommunications Industry Association (“TIA”) help define many U.S. standards, such as IS-41, which is a MAP standard. Analogously, the CCITT and ITU help define international standards, such as GSM-MAP, which is an international MAP standard. Information about these standards is well known and may be found from the relevant organizing bodies as well as in the literature, see, e.g., Bosse, SIGNALING IN TELECOMMUNICATIONS NETWORKS (Wiley 1998).
To deliver a call from an MS 114, a user dials the number and presses “send” on a cell phone or other MS. The MS 114 sends the dialed number indicating the service requested to the MSC 110 via the BS 107. The MSC 110 checks with an associated VLR 116 (described below) to determine whether the MS 114 is allowed the requested service. The serving MSC routes the call to the local exchange of the dialed user on the PSTN 120. The local exchange alerts the called user terminal, and an answer back signal is routed back to the MS 114 through the serving MSC 110 which then completes the speech path to the MS. Once the setup is completed the call may proceed.
To deliver a call to an MS 114, (assuming that the call originates from the PSTN 120) the PSTN user dials the MS's associated phone number. At least according to U.S. standards, the PSTN 120 routes the call to the MS's home MSC (which may or may not be the MSC serving the MS). The MSC then interrogates the HLR 118 to determine which MSC is currently serving the MS. This also acts to inform the serving MSC that a call is forthcoming. The home MSC then routes the call to the serving MSC. The serving MSC pages the MS via the appropriate BS. The MS responds and the appropriate signaling links are set up.
During a call, the BS 107 and MS 114 may cooperate to change channels or BTSs 102, if needed, for example, because of signal conditions. These changes are known as “handoffs,” and they involve their own types of known messages and signaling.
FIG. 2 shows in more detail the signaling and user traffic interfaces between a BS 107 and an MSC 110 in a CDMA mobile network. The BS 107 communicates signaling information using an SS7-based interface for controlling voice and data circuits known as the “A1” interface. An interface known as “A2” carries user traffic (such as voice signals) between the switch component 204 of the MSC and the BS 107. An interface known as “A5” is used to provide a path for user traffic for circuit-switched data calls (as opposed to voice calls) between the source BS and the MSC. Information about one or more of A1, A2, A5 may be found in CDMA Internetworking—Deploying the Open-A Interface, Su-Lin Low, Ron Schneider, Prentice Hall, 2000, ISBN 0-13-088922-9.
Mobile communications providers are supplying newer services, e.g., “data calls” to the Internet. For at least some of these services, MSCs are not cost effective because they were primarily designed for voice calls. Integration of new services into the MSC is difficult or infeasible because of the proprietary and closed designs used by many MSC software architectures. That is, the software logic necessary to provide the services is not easy to add to the MSC 110. Often, a switch adjunct is used to provide such services. For example, an Inter-Working Function (“IWF”) is an adjunct to route a data call to the Internet. Either approach—integrating functionality into the MSC or adding a trunk-side adjunct—involves the MSC in the delivery of service. Integrating new services via MSC design changes or through trunk-side adjuncts can increase network congestion at the MSC and consume costly MSC resources.
Data calls typically make use of the Internet, which is an example of a packet-switching medium. A packet-switching medium operates as follows. A sequence of data is to be sent from one host to another over a network. The data sequence is segmented into one or more packets, each with a header containing control information, and each packet is routed through the network. A common type of packet switching is datagram service, which offers little or no guarantees with respect to delivery. Packets that may belong together logically at a higher level are not associated with each other at the network level. A packet may arrive at the receiver before another packet sent earlier by the sender, may arrive in a damaged state (in which case it may be discarded), may be delayed arbitrarily (notwithstanding an expiration mechanism that may cause it to be discarded), may be duplicated, and may be lost.
At least one wireless Internet system has been proposed that provides reliable access to tens of megahertz of bandwidth across a wide geographic area, using local wireless transceiver technology (e.g., in a nanocell system). In contrast to the cellular wireless voice system, which relies on tens or hundreds of cells in a region, the local wireless transceiver system relies on thousands or tens of thousands of transceivers in the region. In such a system, each transceiver may cover, e.g., 0.05 square kilometers, which is about one-hundredth the coverage of a conventional cell. High spatial reuse of the radio frequency (RF) spectrum allows the local wireless transceiver system to accommodate many more active devices at a given data rate than a conventional cell system. In addition, since users are closer to access points, the local wireless transceiver system accommodates lower-power transmissions. The local wireless transceiver system can support large numbers of devices, running at high speeds, with relatively little drain on the devices' batteries.
For example, in a citywide local wireless transceiver system network of 10,000 transceiver access points (cell centers), if each point provides its users with 1-Mb/s collective throughput, 10 active devices per transceiver can be supported at 100 kb/s each, which amounts to 100,000 active devices in the city. If each device is active 10 percent of the time, such a network can support a million devices, although some accounting would need to be made for bandwidth consumed by overhead for channel access, handoffs, and any provision for asymmetric traffic (e.g., in which more bits flow toward a device than from it).
Each local wireless transceiver system access point may be or resemble access points for wireless local area network (LAN) technology such as IEEE 802.11. An asynchronous digital subscriber line (ADSL), or a cable modem line may be used to provide a link between each access point and the Internet (a wireless link may be used as well or instead). With respect to the siting of access devices, since each device requires electrical power and is preferably elevated for adequate radio frequency coverage, sites on utility poles and buildings are typical candidates, with the high-speed neighborhood Internet access infrastructure serving as a backbone.
In communications systems as described above, it is desirable to provide redundancy without excessive complexity.